We have tried some cpu mining with Vmware server.
and i must say the performance loss was minimal . we used a i-7 core with virtualisation options

Gpu mining in our case was a no no because vmware server does not support more than 32mb of video memory for Ati/Amd cards.
I personally think this will lead to a huge performance impact. as virtualisation for videocards is not fully developed/supported yet.